Now I think she was like the Dark Lugia on Pokémon XD Gale of Darkness. He was corrupted SO MUCH, he couldn't return to his normal self with the same treatment that the other Shadow Pokémon. Other Pokémon could be "purified" just with your trust and love, and by having contact with other Pokémon. But he needed something more, something much more powerful. He needed the combined love and friendship of SIXTY POKÉMON AT ONCE aided by the power of science. Only then, he returned to be a normal Pokémon, no more governed by hate and evil, but by love and friendship.